1. Field of the Invention
The present invention relates to a method and apparatus for uplink scheduling in a communication system.
2. Description of the Related Art
In general, an UpLink (UL) scheduler of a Base Station (BS) is made aware of the existence of data in a buffer of a User Equipment (UE) by receiving a Buffer State Report (BSR) from the UE. If the UL scheduler fails to receive the BSR from the UE, it cannot determine the existence or absence of data in the buffer, even though the buffer of the UE has the data. As a result, the UL scheduler does not allocate resources for data transmission to the UE.
Since the BSR is also a kind of data, the BSR can be transmitted to the UL scheduler only when the UL scheduler allocates resources for transmission of the BSR. Accordingly, if the UL scheduler does not allocate resources for BSR transmission, the UE may not transmit the BSR to the UL scheduler and thus may not be allocated resources for data transmission. That is, the UE may be placed in a deadlock state. This phenomenon is called BSR stall. The reason for causing the BSR stall phenomenon is that the UE and the UL scheduler operate in the following manner.
Firstly, the UL scheduler estimates the buffer length of the UE based on a BSR received from the UE and extracts the amount of data that the UE has transmitted successfully from its buffer based on the estimated buffer length. Herein, the size of the buffer is assumed to be a buffer length.
Secondly, if the estimated buffer length of the UE is ‘0’, the buffer of the UE is determined to be empty and the UL scheduler excludes the UE from a UL scheduling list that lists UEs to be scheduled.
Thirdly, if new data to be transmitted to the BS is generated before the buffer of the UE is empty, that is, before the estimated buffer length is ‘0’ after the UE transmitted the previous BSR to the UL scheduler, the UE does not trigger a regular BSR.
Finally, if no BSR has been transmitted to the UL scheduler during a predetermined BSR transmission period, the UE triggers a periodic BSR. Since the UE does not trigger a Scheduling Request (SR) in this case, the UL scheduler does not know whether the UE's buffer has a BSR to be transmitted. In addition, due to no transmission of a BSR to the UL scheduler during the predetermined BSR transmission period, the UE is excluded from the UL scheduling list and thus not allocated resources for BSR transmission.
According to a standard, it is regulated that a regular BSR is triggered when a UE does not transmit a BSR until expiration of a timer set to a predetermined timer value. Although the BSR stall phenomenon may be mitigated through this operation, a BSR retransmission time to which the timer is set is at least 320 Transmission Time Intervals (TTIs) and thus data buffered in the buffer of the UE is transmitted after the time delay of 320 TTIs unless the UL scheduler performs a certain operation.